Surface-bonded hybrid organic-inorganic polymer coatings and monolithic beds are popular sorbents for use in analytical microextraction. These systems display high chemical stability and offer a diverse array of extracting phases for solvent-free analytical sample preparation. The availability of a wide variety of sol-gel precursors and sol-gel active organic polymers allow facile synthesis of advanced material systems with unique selectivity, enhanced extraction sensitivity and high thermal, mechanical and solvent stability. These sol-gel derived hybrid organic-inorganic advanced material systems have been shown to be effective in solvent free/solvent minimized sample preparation for a wide variety of analytes with biological, environmental, clinical, toxicological, food, pharmaceutical, bio-analytical, and forensic significance.
Sol-gel technology for the preparation of solid phase microextraction (SPME) sorbents has solved many limitations of conventional coatings. Sol-gel coatings chemically bond to many different substrates, such as silica, when the gel is formed from the sol solution in the presence of the substrate. Because of the wide variety of possible sol components, sol-gel technology allows the synthesis of a large number of sorbents for SPME and similar microextraction techniques (e.g., capillary microextraction, stir bar sorptive extraction) with large surface area, unique selectivity, and high thermal and solvent stability. Sol-gel monolithic beds are capable of achieving very high sample pre-concentration factors. The versatility of sol-gel technology allows the creation of surface-bonded sorbent coatings on unbreakable fiber materials (e.g., Ni—Ti, stainless steel, titanium, and copper) and also on substrates of different geometrical formats such as planar SPME (PSPME), and membrane SPME (MSPME). Sol-gel technology is adaptable to forming multi-component materials that have customized surface morphologies, selectivities and affinities of the sorbent. A wide variety of sol-gel silica, titania, zirconia, alumina, and germania-based precursors are commercially available. Additionally, a wide range of sol-gel reactive organic ligands are available to design hybrid organic-inorganic sol-gel coatings that can be used to target a particular analyte or sample matrix with improved selectivity, sensitivity, extraction phase stability and performance.
There remains a strong need for solvent free/solvent minimized microextraction devices that permit the acquisition of very low concentrations of analytes that are present in a wide range of environments. Most microextraction devices are suited to a particular type of environment, and are often poorly suited for other environments. For example, some microextraction devices are well suited to sample air or other gases while others are suited for extraction from water or other liquids. Few are microextraction devices that can be easily adapted for sampling a solid surface. In addition, the limitation inherent to the geometric configurations of microextraction devices (smaller substrate surface area in both fiber and in-tube format) does not allow using high amount of sorbent materials for extraction. The physical immobilization of polymeric materials on the substrate surface in microextraction devices limit their exposure to high temperature for thermal desorption and to organic solvents for solvent mediated desorption. As a result, many compounds with high boiling points and high polarity are still beyond the reach of microextraction devices. Furthermore, the microextraction devices are not recommended to make direct contacts with the sample matrix when it contains high volume of particulates, debris or other matrix interferences (e.g., protein, tissues, fat molecules) that may cause irreversible damage to the sorbent coating.